Imagery is what the author uses to appeal to your senses. So as you read the poem, highlight where you read things about sight, feel, hear, taste, or smell. The main idea of the poem is to describe the many events that occured in Flanders Fields. So, what was heard, seen, felt, tasted or smelled while in the fields? Larks were heard singing, felt the dawn and sunset, see the poppies blow...this textual evidence helps the reader to imagine what has happened in the fields even though you were not there. That is how the main topic was developed through imagery.